The present invention is related to methods and compositions useful to improve the polymerization rate and molecular weight of polystyrene, and more particularly relates to methods and compositions useful to improve the polymerization rate, molecular weight, and rubber phase properties of high impact polystyrene.
High impact polystyrene (HIPS) is an example of graft polymer formation between polystyrene and polybutadiene (rubber). A wide variety of peroxy compounds is known from the literature as initiators for the production of styrenic polymers. Commercially available initiators for polymer production may be classified in different chemical groups, which include diacylperoxides, peroxydicarbonates, dialkylperoxides, peroxyesters, peroxyketals, and hydroperoxides. Certainly, there are ongoing efforts within the industry to improve the polymerization rates of styrenic monomers, polymer molecular weights/molecular weight distributions, and to improve the rubber phase properties (i.e. rubber particle size, increase grafting, % gel, swell index) of HIPS. Attaining these improvements would allow for enhanced production economics and resin physical properties (e.g. improved stiffness/impact strength balance).
Peroxides and hydroperoxides undergo at least four reactions in the presence of monomers or hydrocarbons with double bonds. These reactions are: 1) chain transfer, 2) addition to monomer, 3) hydrogen abstraction, and 4) recombination, often called a cage effect.
Hydroperoxides have been shown to undergo induced decomposition reactions, in which a polymer radical (xcx9cxcx9cP*) will react with the initiator as shown below. This reaction is basically a chain transfer reaction and the reaction should be amenable to the well-known chain transfer equations. Radicals obtained from peroxide initiators (RCOO*) can also abstract a hydrogen from the hydroperoxide.
RCOO* or xcx9cxcx9cP*+RCOOHxe2x86x92xcx9cxcx9cPH+ROO*
Baysal and Tobolsky (Journal of Polymer Science, Vol. 8, p. 529 et seq., (1952), incorporated by reference herein) investigated the chain transfer of polystyryl radicals to t-butylhydroperoxide (t-BHP), cumyl hydroperoxide (CHP), benzoyl peroxide (Bz2O2), and azobisisobutyronitrile (AIBN). AIBN and benzoyl peroxide give the classical linear correlations between rate and 1/DP (Degree of Polymerization) indicating no chain transfer to initiators. The hydroperoxides, however, show significant levels of chain transfer.
A. I. Lowell and J. R. Price (Journal of Polymer Science, Vol. 43, p. 1, et seq. (1960), incorporated by reference herein) also showed that polystyryl radicals undergo considerable chain transfer with bis(2,4-dichloro) benzoyl peroxide as compared to dilauroyl peroxide.
The transition metal catalyzed peroxidation of the pendant allylic functionality in ethylene-propylene-diene monomer (EPDM) rubbers with tertiary butyl hydroperoxide results in elastomeric, high polymer peroxides, according to B. Dean in xe2x80x9cGraft Copolymers from Peroxidized EPDM Rubber,xe2x80x9d Journal of Applied Polymer Science, Vol. 32, pp. 5619-5625 (1986), incorporated by reference herein. The peroxidized EPDM rubbers are useful as free radical initiators for the polymerization and grafting of vinyl monomers in the preparation of comb-type structures. The grafted EPDM rubbers are efficient impact modifiers for thermoplastic resins so long as the polymer grafted onto the EPDM is identical to or is thermodynamically miscible with the composition of the thermoplastic resin.
It would be desirable if methods could be devised or discovered to accelerate the polymerization rate of HIPS, and/or increase the grafting that occurs during this polymerization, as well as to improve other properties of HIPS.
There is provided, in one form, a method for polymerizing at least one vinylaromatic monomer in the presence of a sequential polyperoxide initiator, and then (b) recovering a polymerized product.
In another embodiment of the invention, there are provided monomer compositions that include sequential polyperoxide initiator, and polymerized products made by the method described above.